DC-DC converters are used in a wide range of applications to provide a regulated output voltage from a battery or other power source. Switch-mode or switching DC-DC converters use an energy storage device, such as an inductor, to store the input energy and switches to selectively couple the energy storage device to the output.
Common switching converter topologies include Buck, Boost, Buck-Boost, —, SEPIC, Cúk, half bridge, and full bridge to name a few. In Buck converters, the output voltage is lower than the input voltage, in Boost converters, the output voltage is higher than the input voltage, and in Buck-Boost converters, the output voltage can be either higher or lower than the input voltage but ideally is relatively close to the input voltage. Buck-Boost converters are advantageous in applications with widely varying input voltage ranges, such as in automobiles where the battery voltage can vary widely between start up conditions of a few volts to spikes as high as 40 or 50 volts under certain load conditions, since the converter can be configured to operate in a Buck mode, a Boost mode, or a Buck-Boost mode of operation.
FIG. 1 illustrates a conventional Buck-Boost converter 10 in which an inductor 14 is selectively coupled to an input voltage source 18 to store energy when switches Q1 and Q2 are both on or decoupled from the input voltage source to transfer energy to the load Ro at the converter output 22 through diodes D1 and D2 when switches Q1 and Q2 are both off in a Buck-Boost mode of operation. The converter 10 can be configured to operate in a purely Buck mode of operation by keeping switch Q2 off, as may be desirable when the input voltage is significantly higher than the desired output voltage or in a purely Boost mode of operation by keeping switch Q1 off, as may be desirable when the input voltage is significantly lower than the desired output voltage.
A control circuit 20 controls the duty cycle of conduction of the switches Q1 and Q2 based on feedback from the converter output 22 in order to provide the output voltage Vout at a desired, predetermined regulated voltage level. For example, the control circuit 20 may include an error amplifier 24 to generate an error signal Vc, 26 based on a difference between the converter output 22 and a reference signal Vref. The control circuit 20 may further include a comparator 28 to compare the error signal Vc to a Pulse Width Modulation (PWM) ramp signal to generate a switch control signal 30 for coupling to the gate terminal of stitches Q1 and Q2. In some converters, the switches Q1 and Q2 may be independently controlled, in which case an additional comparator 34 may be provided to generate a control signal 36 to the gate terminal of switch Q2. Such independent control of the switches Q1 and Q2 permits the converter 10 to be configured in a Buck mode of operation or in a Boost mode of operation and also permits a third phase of Buck-Boost operation (i.e., operation with one of the switches Q1 and Q2 on and the other one off) as may be desirable to reduce losses and improve efficiency when transitioning between switch phases.